1. Field of the Invention
This invention relates to a multi cylinder internal combustion engine having apparatus for purifying exhaust gases, and in particular to such engines for use in automobiles.
2. Description of the Prior Art
The large number of vehicles with internal combustion engines has caused air pollution problems due to the vehicle exhaust, and as a result various types of exhaust purification means have been proposed. They are, however, defective in view of durability, costs, size of the apparatus, purification power and the like.
In vehicle internal combustion engines, each of the cylinders is generally designed so that the air-fuel ratio to be supplied is kept as uniform as possible. However, during a partial load condition the concentration of NOx in the exhaust gas is high at the air-fuel ratio at which the fuel consumption can be reduced to minimum. During a heavy load condition where the throttle is opened near to its extreme opening position, the concentration of CO and HC left as unburnt gases in the exhaust is high since the enriched air-fuel mixture in the cylinders undergoes incomplete combustion. This often results from low temperature at the inner wall of the cylinders or the like.
As is well-known, while the concentrations of CO and HC in the exhaust gas can be reduced through effective combustion at a high temperature by supplying sufficient amounts of air, the NOx concentration is increased as the combustion temperature goes higher. The combustion temperature should, therefore, be lowered in order to reduce the NOx concentration.
Thus, the following three factors are present in typical internal combustion engines:
(1) NOx concentration is reduced both at lower air-fuel ratio and at higher air-fuel ratio as compared to that ratio resulting in excellent combustion; PA0 (2) CO and HC concentrations are increased at lower air-fuel ratio; and PA0 (3) CO and HC concentrations are reduced and oxygen concentration is increased at higher air-fuel ratio provided that there are no misfirings. (These three normal factors can be seen in FIG. 3)